The reduceOp caught my eye in your API. What uses have you imagined/tried for changing the density accumulation function from x + y?
When the number of the bins in the data is not an exact integer multiple of the number of bins in the histogram, adjacent data bins can get mapped to the same histogram bin, resulting in e.g. 2x the data volume in some rows/columns of the histogram.
When a bit of loss of fidelity is acceptable the two solutions I've used are to render the histogram at an exact factor of the number of data bins then set the canvas dimensions to the desired size (relying on the browser to downsample the resulting image), or to use `max` as the reduceOp and render the histogram directly at the intended size.